Tuesday, March 17, 2015

Back focus while using AF-A and AF-C on Sony a6000 and a5100

These cameras came with focus mode set to AF-A by default. In this mode, the camera is supposed to use AF-S and, if it sees object moving, switch automatically to AF-C. Cool, I thought, saving the trouble switching manually between AF-S and AF-C, especially on the a5100 that lacks of a fast menu (Fn). So I left them in AF-A.

What a mistake! I got so many shots that were out of focus, such as this one (16-50 PZ at f/5.0):

So I downloaded a focus test chart from Jeffrey Friedl and investigated into the focus issue. Turned out, the camera always do back focus while using AF-A or AF-C modes. I tested with a6000 + SEL50F18 and a5100 + SEL35F18 and both behaved exactly the same.

This back focus issue is easier to notice at f/2.2 and f/2.5, and not so noticeable at f/2.0 and f/1.8. It appears to be more severe in AF-C mode than AF-A. Above photo was taken in AF-A mode.

Once I switch to AF-S, focusing becomes normal:

Although I only present two test charts here, I've done lots of tests to reach the pattern which is totally repeatable. Hopefully Sony will address this issue with a new firmware update. Until then, we'd better avoid AF-A and AF-C and live with AF-S.

[update] I did more tests, now I can safely say:
  1. For non-moving objects, AF-C is not as accurate, dependable and consistent as AF-S. This is normal, because AF-C is ought for tracking moving objects.
  2. AF-A may use either AF-S or AF-C algorithm for non-moving objects. This is not right, because AF-A should always be identical to AF-S, not AF-C, if everything is still.
So, never use AF-A - always use either AF-S or AF-C for your specific shooting situation.

The following photo and video show that AF-A picked improperly AF-C over AF-S:

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